Medium-entropy alloy having excellent cryogenic properties

ABSTRACT

Disclosed is a medium-entropy alloy, which is further improved in cryogenic mechanical properties of an existing FCC-based high-entropy alloy and is capable of ensuring price competitiveness, the medium-entropy alloy including 6 to 15 at % of Cr, 50 to 64 at % of Fe, 13 to 25 at % of Co, 13 to 25 at % of Ni, and the remainder of inevitable impurities, wherein the medium-entropy alloy includes a metastable FCC phase, whereby deformation-induced phase transformation from an FCC phase into a BCC phase occurs upon plastic deformation of the alloy, thus manifesting excellent cryogenic mechanical properties.

TECHNICAL FIELD

The present invention relates to a medium-entropy alloy (MEA) havingexcellent cryogenic mechanical properties, in which inexpensive Fe isincluded in an amount of 50 at % or more to thus exhibit high pricecompetitiveness, and moreover, in which face-centered cubic (FCC) andbody-centered cubic (BCC) phase stability may be adjusted throughcontrol of alloying elements to thus cause deformation-induced phasetransformation during cryogenic deformation, thereby realizing excellentcryogenic mechanical properties.

BACKGROUND ART

A high-entropy alloy (HEA) is a multielement alloy obtained by alloyingfive or more constituent elements at a similar ratio without a majorelement of the alloy. A high-entropy alloy is a metal material having asingle-phase structure, such as a face-centered cubic (FCC) phase or abody-centered cubic (BCC) phase, without forming intermetallic compoundsor intermediate phases, due to high entropy of mixing in the alloy.

In particular, a Co—Cr—Fe—Mn—Ni-based high-entropy alloy has excellentcryogenic properties, high fracture toughness and high corrosionresistance, and is thus receiving attention as a material suitable foruse in extreme environments.

Two important factors in designing such a high-entropy alloy are thecomposition ratio of the constituent elements of the alloy and theconfigurational entropy of the alloy system.

Here, the composition ratio of the high-entropy alloy is discussedfirst. A high-entropy alloy has to be composed of at least fiveelements, and the fraction of each of the constituent elements of thealloy is set to the range of 5 to 35 at %. Furthermore, when anotherelement is added in the production of a high-entropy alloy, in additionto the main alloying elements, the amount thereof should be 5 at % orless.

Also, alloys are typically divided into high-entropy alloys,medium-entropy alloys (MEAs), and low-entropy alloys (LEAs), dependingon the configurational entropy (ΔS_(conf)) of the composition ofalloying elements, and are classified according to the conditions ofEquation 2 below based on the configurational entropy value determinedby Equation 1 below.

$\begin{matrix}{{\Delta \; S_{conf}} = {{- R}{\sum\limits_{i = 1}^{n}{X_{i}\ln \; X_{i}}}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

(R: Gas constant, X_(i): mole fraction of i element, n: number ofconstituent elements)

ΔS _(conf)≤1.0·R(LEAs),

1.0·R≤ΔS _(conf)≤1.5·R(MEAs),

1.5·R≤ΔS _(conf)(HEAs)  [Equation 2]

For a Co₂₀Cr₂₀Fe₂₀Mn₂₀Ni₂₀ (at %) alloy, which is a representativecryogenic FCC-based high-entropy alloy, the alloying elements that areadded are expensive, resulting in low price competitiveness.Accordingly, despite the excellent cryogenic properties thereof, theabove alloy makes it difficult to serve as a replacement for existingsteel materials in marine plants, LNG container materials, cryogenictanks, and ship/marine materials.

Therefore, the industrialization of high-entropy alloys is essentiallyrequired to ensure price competitiveness through control of alloyingelements and also to realize excellent cryogenic properties.

CITATION LIST

-   (Patent Document) U.S. Patent Application Publication No.    2002/0159914-   (Non-Patent Document) 1. B. Gludovatz, et al., “A fracture-resistant    high-entropy alloy for cryogenic applications”, Science, 345 (2014)    1153-1158.

DISCLOSURE Technical Problem

Accordingly, an objective of the present invention is to provide amedium-entropy alloy, which is capable of exhibiting superior mechanicalproperties by causing deformation-induced phase transformation atcryogenic temperatures, as well as ensuring price competitiveness bydeveloping an alloy including reduced amounts of expensive alloyingelements, in lieu of a conventional Co—Cr—Fe—Mn—Ni-based alloy.

Technical Solution

In order to accomplish the above objective, the present inventionprovides a medium-entropy alloy, comprising 6 to 15 at % of Cr, 50 to 64at % of Fe, 13 to 25 at % of Co, 13 to 25 at % of Ni, and the remainderof inevitable impurities.

Moreover, the medium-entropy alloy according to an embodiment of thepresent invention includes a metastable FCC phase at room temperatureand causes deformation-induced phase transformation from the metastableFCC phase into a BCC phase upon cryogenic deformation, and is thusimproved in mechanical properties.

Advantageous Effects

According to the present invention, a medium-entropy alloy is configuredsuch that the amount of Fe, which is an inexpensive alloying element, isincreased to the range of 50 to 64 at %, thus reducing the amounts ofexpensive Co, Cr, and Ni elements that are added, thereby ensuring pricecompetitiveness, and moreover, the medium-entropy alloy has superiorproperties including tensile strength of 1024 MPa or more and elongationof 47% or more at a cryogenic temperature (77 K).

Moreover, according to an embodiment of the present invention, themedium-entropy alloy includes a metastable FCC phase at room temperature(298 K) and causes deformation-induced phase transformation from themetastable FCC phase into a BCC phase upon deformation at a cryogenictemperature to thus exhibit strengthening effects, ultimately obtainingfurther improved cryogenic mechanical properties.

DESCRIPTION OF DRAWINGS

FIG. 1 shows the results of measurement of X-ray diffraction (XRD) ofCo—Cr—Fe—Ni-based medium-entropy alloys of Comparative Examples 1 and 2and Examples 1 to 4 according to the present invention;

FIG. 2 shows the results of tensile testing at room temperature (298 K)of the Co—Cr—Fe—Ni-based medium-entropy alloys of Comparative Examples 1and 2 and Examples 1 to 4 according to the present invention;

FIG. 3 shows the results of tensile testing at a cryogenic temperature(77 K) of the Co—Cr—Fe—Ni-based medium-entropy alloys of ComparativeExamples 1 and 2 and Examples 1 to 4 according to the present invention;and

FIG. 4 shows the analytical results of electron backscatter diffraction(EBSD) for phase transformation upon deformation at room temperature andat a cryogenic temperature of the Co—Cr—Fe—Ni-based medium-entropy alloyof Example 3 according to the present invention.

BEST MODE

Hereinafter, a detailed description will be given below of preferredembodiments of the present invention with reference to the appendeddrawings, but the present invention is not limited to the followingexamples. Accordingly, those skilled in the art will appreciate thatvarious modifications are possible, without departing from the spirit ofthe invention.

The present inventors have performed thorough study in order to obtainexcellent mechanical properties in cryogenic environments whileincreasing price competitiveness of high-entropy alloys having superiormechanical properties in cryogenic environments, and thus haveascertained that the amount of Fe, which is an inexpensive element, isremarkably increased to the range of 50 to 64 at % compared toconventional high-entropy alloys, and the amounts of alloying elementsother than Fe are adjusted, whereby deformation-induced phasetransformation may occur during deformation due to changes in FCC andBCC phase stability, resulting in excellent cryogenic mechanicalproperties.

In particular, the present inventors have revealed that when an alloy isdesigned so as to include an FCC phase in a metastable state at roomtemperature, deformation-induced phase transformation from the FCC phasein a metastable state into a BCC phase may occur during deformation in acryogenic environment, thereby further improving cryogenic mechanicalproperties, which culminates in the present invention.

In the present invention, when the phase in a metastable state istransformed into a phase in a stable state at the correspondingtemperature through deformation-induced phase transformation duringplastic deformation, it is judged to be a metastable phase. All of thesephases are defined as a metastable phase.

The medium-entropy alloy according to the present invention has an alloycomposition comprising 6 to 15 at % of Cr, 50 to 64 at % of Fe, 13 to 25at % of Co, 13 to 25 at % of Ni, and the remainder of inevitableimpurities.

Furthermore, the medium-entropy alloy according to the present inventionincludes a metastable FCC at room temperature, and enables occurrence ofdeformation-induced phase transformation from the metastable FCC phaseinto a BCC phase upon deformation.

If the amount of chromium (Cr) is less than 6 at %, the FCC phase isstabilized. On the other hand, if the amount thereof exceeds 15 at %,the BCC phase is stabilized. Hence, the amount of Cr preferably falls inthe range of 6 to 15 at %. Also, since the formation of the metastableFCC phase is more favorable in terms of improving cryogenic mechanicalproperties, the amount of chromium (Cr) more preferably falls in therange of 7.5 to 12.5 at %.

If the amount of iron (Fe) is less than 50 at %, the FCC phase isstabilized. On the other hand, if the amount thereof exceeds 64 at %,the BCC phase is stabilized. Hence, the amount of Fe preferably falls inthe range of 50 to 64 at %. Since the formation of the metastable FCCphase is more favorable in terms of improving cryogenic mechanicalproperties, the amount of iron (Fe) more preferably falls in the rangeof 55 to 62.5 at %.

If the amount of cobalt (Co) is less than 13 at %, the FCC phase isstabilized. On the other hand, if the amount thereof exceeds 25 at %,the BCC phase is stabilized. Hence, the amount of Co preferably falls inthe range of 13 to 25 at %.

If the amount of nickel (Ni) is less than 13 at %, the BCC phase isstabilized. On the other hand, if the amount thereof exceeds 25 at %,the FCC phase is stabilized. Hence, the amount of Ni preferably falls inthe range of 13 to 25 at %.

If the amount of at least one selected from among molybdenum (Mo) andaluminum (Al), which is a component that may substitute for cobalt (Co),is less than 13 at %, the FCC phase is stabilized. On the other hand, ifthe amount thereof exceeds 25 at %, the BCC phase is stabilized. Hence,the amount thereof preferably falls in the range of 13 to 25 at %.

If the amount of manganese (Mn), which is a component that maysubstitute for nickel (Ni), is less than 13 at %, the BCC phase isstabilized. On the other hand, if the amount thereof exceeds 25 at %,the FCC phase is stabilized. Hence, the amount thereof preferably fallsin the range of 13 to 25 at %.

In general, an interstitial element such as C or N in a metal alloy issubjected to interstitial solid solution in the metal matrix to thusenhance the strength of the alloy due to solid-solution strengtheningeffects during metal deformation. When at least one element of C and Nis added in an amount of 1 at % or more based on the total at % of thealloy, the FCC phase is stabilized. In order to utilize the effect ofdeformation-induced phase transformation by inducing the metastable FCCphase, it is preferred that the above element be added in an amount ofless than 1 at %.

The inevitable impurities are components other than the above alloyingelements and are unavoidable components that are inevitably incorporatedinto the alloying elements or during the production process.

The medium-entropy alloy may be composed of a metastable FCC phase or acombination of a metastable FCC phase and a BCC phase at roomtemperature. Here, it is preferred that the fraction of the metastableFCC phase be high in the interests of improvements in tensile strengthand elongation. The fraction of the metastable FCC phase is preferably50% or more. However, the fraction of the metastable FCC phase is notnecessarily 50% or more.

Also, the medium-entropy alloy may have tensile strength of 500 MPa ormore and elongation of 50% or more at room temperature (298 K).

Also, the medium-entropy alloy may have tensile strength of 1000 MPa ormore and elongation of 40% or more at a cryogenic temperature (77 K).

Examples 1 to 4

Production of Medium-Entropy Alloy

First, Co, Cr, Fe, and Ni metals having purity of 99.9% or more wereprepared.

The metals thus prepared were weighed in the mixing fractions shown inTable 1 below.

TABLE 1 Metal mixing fraction (at %) Co Cr Fe Ni Example 1 17.50 10.0055.00 17.50 Example 2 16.25 10.00 57.50 16.25 Example 3 15.00 10.0060.00 15.00 Example 4 13.75 10.00 62.50 13.75

The metals prepared in the above fractions were placed in a crucible,heated to 1550° C. and thus melted, and then cast into 150 g of an alloyingot having a cuboid shape having a width of 33 mm, a length of 80 mm,and a thickness of 7.8 mm, using a mold.

In order to remove the oxide formed on the surface of the cast alloy,surface grinding was performed. The thickness of the ground ingot was 7mm.

The surface-ground ingot having a thickness of 7 mm was subjected tohomogenization heat treatment at 1100° C. for 6 hr and then cold rollingto a thickness from 7 mm to 1.5 mm.

Furthermore, the cold-rolled alloy plate was annealed at 800° C. for 10min.

Comparative Examples 1 and 2

Production of Alloy for Comparative Examples

The alloys of Comparative Examples were manufactured in the same manneras in Examples using the components in the amounts shown in Table 2below.

TABLE 2 Metal mixing fraction (at %) Co Cr Fe Ni Comparative Example 114.50 5.00 66.00 14.50 Comparative Example 2 12.50 10.00 65.00 12.50

The alloy ingot was cast in the same manner as in Examples, followed byhomogenization heat treatment at 1100° C. for 6 hr and then cold rollingto a thickness from 7 mm to 1.5 mm in the same manner as in Examples.

Furthermore, the cold-rolled alloy plate was annealed at 800° C. for 10min in the same manner as in Examples.

Component Analysis Results

The actual components of the alloys manufactured in Comparative Examples1 and 2 and Examples 1 to 4 after annealing treatment were analyzedusing EDS. The results are shown in Table 3 below.

TABLE 3 EDS analysis composition (at %) Co Cr Fe Ni Comparative Example1 14.34 5.10 66.29 14.27 Example 1 17.37 10.52 55.58 16.53 Example 216.16 10.21 57.41 16.22 Example 3 14.54 10.68 60.89 13.89 Example 413.55 10.27 62.55 13.63 Comparative Example 2 12.23 10.81 65.31 11.65

As is apparent from Table 3, the actual composition falls slightly outof the range of the initial metal mixing fractions, but may be regardedas almost the same level considering the purity of metals and impuritieswhich may be incorporated during the production process. All of Examplesfell in the composition range of the medium-entropy alloy according tothe present invention, comprising 6 to 15 at % of Cr, 50 to 64 at % ofFe, 13 to 25 at % of Co, and 13 to 25 at % of Ni.

XRD Analysis Results

FIG. 1 shows the results of XRD measurement at room temperature of theannealed alloys of Comparative Examples 1 and 2 and Examples 1 to 4.

XRD measurement was performed after grinding in the order of sandpaperNos. 600, 800 and 1200 and then electrolytic etching in 8% perchloricacid in order to minimize phase transformation due to deformation duringgrinding of a test specimen.

As shown in FIG. 1, Comparative Example 1 was composed of the BCC phase,Examples 1 to 4 were composed mainly of the metastable FCC phase, andComparative Example 2 was composed mainly of the BCC phase and includeda small amount of the FCC phase.

Specifically, when the amount of Fe was increased and the amounts of Coand Ni were decreased, the stability of the FCC phase was deteriorated,and consequently the metastable FCC phase was formed in Examples 1 to 4.In Comparative Examples 1 and 2, in which Fe was added in an amount of65 at % or more, the FCC phase was no longer in the metastable state butbecame unstable, and the BCC phase was relatively stabilized.

Tensile Test Results

The results of tensile testing at room temperature (298 K) and at acryogenic temperature (77 K) of the annealed alloys of ComparativeExamples 1 and 2 and Examples 1 to 4 according to the present inventionare shown in FIGS. 2 and 3 and Table 4 below.

FIGS. 2 and 3 are graphs showing the results of tensile testing at roomtemperature and at a cryogenic temperature, respectively, in which thehorizontal axis designates the engineering strain and the vertical axisdesignates the engineering stress. Based on the graphs of the testresults, the results of analysis of physical properties such as yieldstrength, tensile strength and elongation of Comparative Examples andExamples 1 to 4 are given in Table 4 below.

TABLE 4 Room temperature Cryogenic temperature (77 K) Yield TensileElon- Yield Tensile Elon- Test strength strength gation strengthstrength gation specimen (MPa) (MPa) (%) (MPa) (MPa) (%) Comparative 850975 24 1336 1455 33 Example 1 Example 1 280 550 68 615 1024 126 Example2 274 568 86 543 1164 118 Example 3 226 534 98 526 1508 82 Example 4 228787 67 620 1649 47 Comparative 579 996 26 1110 1516 30 Example 2

As is apparent from FIGS. 2 and 3 and Table 3, the tensile properties atroom temperature of the medium-entropy alloys of Examples 1 to 4according to the present invention exhibited yield strength of 226 to280 MPa, tensile strength of 534 to 787 MPa, and elongation of 67 to98%.

Furthermore, excellent tensile properties at a cryogenic temperature,such as yield strength of 526 to 620 MPa, tensile strength of 1024 to1649 MPa, and elongation of 47 to 126%, were manifested.

In contrast, the tensile properties at room temperature of themedium-entropy alloys of Comparative Examples 1 and 2 were as follows:since the initial crystal structure was mostly composed of a BCCstructure, there were neither strengthening effects nor elongationenhancement effects due to deformation-induced phase transformationbetween tensile deformation at room temperature and tensile deformationat a cryogenic temperature, and tensile yield strength and tensilestrength were high at room temperature and at a cryogenic temperaturedue to the BCC structure, but elongation was low, resulting inbrittleness.

In particular, the alloy of Example 3, including a large amount of theFCC phase in the metastable state, manifested excellent tensileproperties at a cryogenic temperature, such as yield strength of 526MPa, tensile strength of 1508 MPa, and elongation of 82%, which were notpreviously reported.

Additionally, in the medium-entropy alloy of the present invention, evenwhen at least one of Mo and Al substituting for Co was added in the sameamount as Co under the condition that the amounts of Cr and Fe weremaintained, deformation-induced phase transformation occurred duringdeformation, as was expected in the present invention, whereby ductilityand stiffness were observed at a cryogenic temperature.

Also, in the medium-entropy alloy of the present invention, even when Mnsubstituting for Ni was added in the same amount as Ni under thecondition that the amounts of Cr and Fe were maintained,deformation-induced phase transformation occurred during deformation, aswas expected in the present invention, whereby ductility and stiffnesswere observed at a cryogenic temperature.

Furthermore, when at least one of C and N was subjected to solidsolution as an interstitial element in the metal matrix of themedium-entropy alloy of the present invention, it was also confirmedthat the strength of the alloy was increased due to the solid-solutionstrengthening effect.

Deformation-Induced Phase Transformation

FIG. 4 shows the analytical results of EBSD for phase transformation ofthe medium-entropy alloy of Example 3 during deformation at roomtemperature and at a cryogenic temperature according to the presentinvention.

As shown in FIG. 4, the alloy of Example 3 included a very small amountof the BCC phase and was composed mainly of the metastable FCC phasebefore deformation, and the fraction of the BCC phase was remarkablyincreased after deformation at room temperature (298 K) and at acryogenic temperature (77 K). In particular, phase transformation fromthe FCC phase into the BCC phase occurs over the entire region afterdeformation at a cryogenic temperature, and this phase transformationcontributes greatly to the improvement of cryogenic mechanicalproperties, as shown in FIG. 3.

Therefore, with regard to the cryogenic mechanical properties, thefraction of the FCC phase before deformation is preferably set to 50% ormore.

TABLE 5 Before deformation After deformation After deformation (vol %)(298 K) (vol %) (77 K) (vol %) Comparative 91.26 93.96 98.99 Example 1Example 1 0.34 15.07 28.46 Example 2 0.38 20.26 36.12 Example 3 0.4127.68 56.71 Example 4 25.68 62.23 85.08 Comparative 87.81 89.20 94.87Example 2

Table 5 shows the results of ferritescope measurement of the BCC phasefraction (vol %) of the alloys of Comparative Examples 1 and 2 andExamples 1 to 4 according to the present invention, before deformationand after deformation at room temperature and at a cryogenictemperature.

As is apparent from Table 5, the alloys of Examples 1 to 3 included asmall amount of the BCC phase before deformation and were increased inthe fraction of the BCC phase due to phase transformation betweendeformation at room temperature and deformation at a cryogenictemperature. Also, the alloy of Example 4 was relatively increased inBCC phase stability compared to the alloys of Examples 1 to 3, and thusit was confirmed that 25.68 at % of the BCC phase was included beforedeformation and that the fraction of the BCC phase was increased due tophase transformation between deformation at room temperature anddeformation at a cryogenic temperature. The alloys of ComparativeExamples 1 and 2 were very high in BCC phase stability compared to thealloys of Examples 1 to 4, and thus it was confirmed that 91.26 at % and87.81 at % of the BCC phases, respectively, were included beforedeformation and that the fraction of the BCC phase was increased due tophase transformation between deformation at room temperature anddeformation at a cryogenic temperature.

1. A medium-entropy alloy, comprising 6 to 15 at % of Cr, 50 to 64 at %of Fe, 13 to 25 at % of Co, 13 to 25 at % of Ni, and a remainder ofinevitable impurities, wherein deformation-induced phase transformationfrom a face-centered cubic (FCC) phase into a body-centered cubic (BCC)phase occurs upon plastic deformation.
 2. The medium-entropy alloy ofclaim 1, wherein the deformation-induced phase transformation occurs ina metastable FCC phase.
 3. The medium-entropy alloy of claim 1, whereinan amount of the Cr is 7.5 to 12.5 at %.
 4. The medium-entropy alloy ofclaim 3, wherein an amount of the Fe is 57.5% to 62.5 at %.
 5. Themedium-entropy alloy of claim 1, wherein the Co is substitutable with atleast one selected from among Mo and Al.
 6. The medium-entropy alloy ofclaim 1, wherein the Ni is substitutable with Mn.
 7. The medium-entropyalloy of claim 1, wherein at least one of C and N is included in anamount of less than 1 at % based on a total at % of the medium-entropyalloy.
 8. The medium-entropy alloy of claim 6, wherein at least one of Cand N is included in an amount of less than 1 at % based on a total at %of the medium-entropy alloy.
 9. The medium-entropy alloy of claim 1,wherein the deformation occurs at a temperature equal to or lower thanroom temperature (298 K).
 10. The medium-entropy alloy of claim 2,wherein a fraction of the metastable FCC phase is 50% or more.
 11. Themedium-entropy alloy of claim 1, wherein the medium-entropy alloy iscomposed of a combination of a BCC phase and a metastable FCC phase, oris composed of a metastable FCC phase alone.
 12. The medium-entropyalloy of claim 1, wherein the medium-entropy alloy has a tensilestrength of 226 MPa or more and an elongation of 67% or more at roomtemperature (298 K).
 13. The medium-entropy alloy of claim 1, whereinthe medium-entropy alloy has a tensile strength of 1024 MPa or more andan elongation of 47% or more at a cryogenic temperature (77 K).